Holder for a tubular safety fuse
BACKGROUND OF THE INVENTION 
     The invention is directed to a holder for a tubular safety fuse which has contact caps at opposite end faces. Such holders are known for fuses in power supply lines of telephone switching systems. A holder is composed of a back-side receptacle for the fuse and of a front-side closure cap. The receptacle at the back side comprises an axially displaceable contact piece that is resiliently pressed against the contact cap of the back side. A cooperating contact piece on the front side of the holder is anchored in the closure cap. The receptacle further comprises a tubular middle contact into which the cooperating contact piece, fashioned cylindrically, can be introduced in the fashion of a bayonet closure. The middle contact connected to an outgoing line is sufficiently long that it projects into the proximity of the contact cap at the back side. 
     The axially displaceable contact piece is resiliently mounted and secured to a stranded wire that is connected to a terminal pin at the back side for an incoming line. In order to increase the stranded wire&#39;s resiliency, it is wound into a loop. A helical compression spring surrounding the wire has its back end supported against the receptacle and presses the displaceable contact piece against the back contact cap of the fuse. When the fuse is removed, then the displaceable contact piece follows the fuse within the framework of the resiliency of the stranded wire. Before, however, the back contact cap reaches the middle contact, it separates from the contact piece so that a short to the middle contact cannot arise. 
     It has been shown, however, that an excess current that causes the fuse to blow effects a welding of the displaceable contact piece to the contact cap due to arc effects. When the fuse is then replaced, the stranded wire is stretched beyond its normal dimension, so that the displaceable contact piece does not break away from the back contact cap of the fuse until it has already reached the middle contact. In this case, a short with considerable sparking arises, as a result whereof the system could be jeopardized and maintenance personnel in close proximity could be startled. Such fuses, for example, are sometimes situated at the upper side of a switching rack and can usually only be reached with the assistance of ladders. In this case, the said effects could possibly contribute to a falling of an inattentive person changing the fuse. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to fashion the holder such that the risk of a short-circuit is avoided. 
     This object is achieved by the invention of a holder for a tubular safety fuse (1) comprising contact caps (7) at each end, whereby a resiliently axially displaceable contact piece (8) is connected to an incoming line (5) and a fixed cooperating contact piece (11) is connected to an outgoing line (6), whereby the cooperative contact piece (8) is part of a front closure cap (3) of the holder, whereby the outgoing line (6) is connected to a tubular middle contact (12) that surrounds the safety fuse (1) and can be connected closure-like to the cooperating contact piece (11), and whereby the middle contact (12) has its backside projecting up into the proximity of the contact cap (7) connected to the displaceable contact piece (8), characterized in that a longitudinally slotted insulating ring (13) that surrounds the safety fuse (1) and separates it from the middle contact (12) can be introduced into that section of the middle contact (12) that remains free of the cooperating contact piece (11); and in that the insulating ring (13) comprises at least one catch nose (15) that engages behind the back end face of the middle contact (12). 
     The insulating ring is fashioned such that it covers a back region of the middle contact in the direction toward the back contact cap. As a result thereof, the latter can no longer touch the middle contact even in the worst case. The risk of shorts is thus reliably avoided. The longitudinal slot formed in the insulating ring makes it possible to make this ring sufficiently resilient that it can be subsequently introduced into a holder that has already been manufactured or that is already integrated into a rack. As a result of the longitudinal slot, the ring&#39;s diameter can be reduced to such an extent that the catch nose can also be thrust through the middle contact. This catch nose is situated in an angular position in which it projects into a clearance of the receptacle fashioned like a longitudinal slot or clearance, so that no collisions whatsoever with the receptacle derive. An advantageous, outer longitudinal rib at the insulating ring thereby engages into a mating longitudinal slot of the middle contact and thus secures the insulating ring against turning during insertion, so that the catch nose reaches its proper angular position. 
     The inside diameter of the insulating ring is somewhat larger than the outside diameter of the back contact cap, so that it can be easily pushed through. 
     In a development of the invention, two axially parallel guide fingers for the fuse are attached to the back of the insulating ring and one of the guide fingers is arranged as an extension of the catch nose. The guide fingers are arranged such that they project into the longitudinally directed clearances of the receptacle. They embrace the back contact cap in their function position, so that the contact cap is held in a defined position. The insulating ring is also provided with introduction bezels for the contact cap at its front side. 
     The invention shall be set forth in greater detail below with reference to an exemplary embodiment of the invention shown in the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal section through a holder for a tubular safety fuse and an insulating ring; 
     FIG. 2 is another longitudinal section through the holder generally along the line II--II of FIG. 1 without the longitudinal fuse and the insulating ring; 
     FIG. 3 is an offset cross-section through the holder generally along the line III--III of FIG. 1; and 
     FIG. 4 is an axial longitudinal section through the insulating ring of FIG. 3. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to FIGS. 1 through 4, a holder for a tubular safety fuse 1 is composed of a back receptacle 2 and of a front closure cap 3. Terminal pins 4a, 4b for an incoming line 5 and an outgoing line 6 respectively are arranged at the back side of the holder, the terminal pins 4a, 4b being connected to a back contact cap 7a and a front contact cap 7b (not shown) respectively of the safety fuse 1. A signalling line 8a that leads to a display means (not shown) at which an outage of the safety fuse 1 is displayed is connected to a third terminal pin 4c. 
     The receptacle 2 comprises an axially displaceable contact piece 8 that is connected to the terminal pin 4a for the incoming line 5 via a stranded wire 9 wound into a loop. A helical compression spring 10 is pushed over this stranded wire 9 and is supported against a back wall 2a of the receptacle 2 and has its other end supported at the contact piece 8. The front contact cap 7b (not shown) of the safety fuse 1 is thereby pressed against the cooperating contact piece 11 that is secured in the front closure cap 3 and projects tubularly therefrom. 
     The receptacle 2 further comprises a likewise tubular middle contact 12 into which the cooperating contact piece 11 engages in contacting fashion and which is connected to the terminal pin 4b for the outgoing line 6. The middle contact 12 projects into the proximity of the back contact cap 7a. The cooperating contact piece 11 and the middle contact 12 are fashioned like a bayonet closure and are latched to one another secure against axial dislocation. 
     The characteristic of the helical compression spring 10 and the stiffness of the stranded wire 9 are dimensioned such that the back contact cap 7a lifts off from the contact piece 8 when the closure cap 3 is loosened and disengaged, lifting off therefrom before it reaches the middle contact 12. In the case of an excess current that leads to the blowing of the fuse, however, it can occur that the back contact cap 7a welds to the contact piece 8 as a consequence of arc effects. When the safety fuse 1 is pulled out, the contact piece 8 then adheres to the back contact cap 7a and is pulled along to such an extent that the back contact cap 7a is pulled into the region of the middle contact 12 before the contact piece 8 breaks free. 
     An insulating ring 13 is introduced into the middle contact in the extension of the cooperating contact piece 11, this insulating ring 13 being arranged between the safety fuse 1 and the middle contact 12 and preventing a direct contact of the back contact cap 7a with the middle contact 12 before the breaking of the contact piece 8 from the back contact cap 7a. The insulating ring 13 comprises a longitudinal slot 14 and a catch nose 15 that engages behind the middle contact 12 at its back side edge 12a and thus prevents the insulating ring 13 from being pulled back out from the receptacle 2. The longitudinal slot makes it possible to compress the insulating ring to such an extent while being introduced into the middle contact 12 that the catch nose 15 can also be plugged through without damage. It thereby becomes possible to retrofit finished holders built into switching systems with the insulating rings 13. 
     The inside diameter of the insulating ring 13 is dimensioned such that the safety fuse 1 can be pushed therethrough with little play. 
     Axially parallel guide fingers 16 that are attached to the insulating ring 13 at a back side thereof (a side toward the back wall 2a) surround the back contact cap 7a of the introduced safety fuse 1 so that the latter is precisely aligned to the contact piece 8. The guide fingers 16 are arranged such that they project into longitudinally directed clearances 17 for the terminal pins 4b, 4c. One of the guide fingers 16 and the catch nose 15 are joined to one another of one piece in a space-saving fashion. The insulating ring 13 also comprises an outer longitudinal rib 18 that projects into a longitudinal slot of the middle contact 12 and thereby secures the insulating ring 13 in its angular position upon insertion into the middle contact 12, so that the catch nose 15 and the guide fingers 16 can be introduced without difficulty into the clearances 17. 
     Although the present invention has been described with reference to a specific embodiment, those of skill in the art will recognize that changes may be made thereto without departing from the scope and spirit of the invention as set forth in the appended claims.